The present invention relates to a method and apparatus for preparing gypsum products (i.e., products comprising calcium sulfate dihydrate) from starting materials comprising calcined gypsum (i.e., calcium sulfate hemihydrate) and water. More particularly, the present invention relates to an improved valve on a conduit located downstream of the slurry mixer and typically used to supply agitated gypsum slurry to a wallboard production line. The basic technology of gypsum wallboard manufacture is disclosed in U.S. Pat. Nos. 1,500,452; 2,207,339 and 4,009,062 all of which are incorporated by reference herein. The present apparatus provides an improved flow of slurry from the dispensing system which enhances the uniform smoothness of the gypsum slurry at the wallboard production line.
It is well known to produce gypsum products by uniformly dispersing calcined gypsum in water to form a slurry and then casting the slurry into a desired shaped mold or onto a surface and allowing the slurry to set to form hardened gypsum by reaction of the calcined gypsum (calcium sulfate hemihydrite or anhydrite) with the water to form hydrated gypsum (calcium sulfate dihydrate).
A gypsum wallboard mixer typically includes a housing defining a mixing chamber with inlets for receiving calcined gypsum and water, among other additives well known in the art. The mixer includes an impeller or other type of agitator for agitating the contents to be mixed into a mixture or slurry. The discharge gate or extractor controls the flow of slurry from the mixer to the dispensing system.
Slurries exhibiting certain viscosities or other properties require differing amounts of materials, additives, entrained air, and the like, and may also require different processing times and equipment. In setting slurry requiring a low amount of entrained air, it is known to use a “pinch-type” valve that squeezes an elastomeric conduit carrying the slurry. Squeezing down on the conduit reduces the orifice of the conduit, which in turn, increases the pressure drop through the orifice, increases the back pressure, increases the volume of slurry in the mixer, and accelerates the flow through the conduit. This will result in a smoother, less air entrained, and more desirable slurry for certain applications.
Prior apparatuses for addressing some of the operational problems associated with dispensing a smooth, setting slurry include a “pinch-type” valve which can be mechanically operated or operated by compressed air or hydraulics to “pinch” down on the elastomeric conduit. However, the pinch valve deforms the conduit into a flattened or rectangular orifice which is susceptible to plugging from prematurely setting slurry in the flow, particularly in the corners of the orifice, where the flow velocity is lower. Further, the inlet and outlet orifices of the pinch valve are abrupt because pressure is exerted on the conduit in a substantially singular plane and does not allow for a gradual transition. Such abrupt entrances and exits can lead to further plugging of slurry producing equipment, which causes costly downtime for repairs.
Muscle valves, consisting of a hydraulic chamber around a thick elastomeric sleeve, provide a round orifice but an abrupt flow channel since the pressure is exerted in a substantially singular plane or at one point along the conduit. Further, the muscle valves frequently are massive in size which limits the operator's access to the orifice for clearing a plug or buildup, or for general observation of the flow of slurry through the channel itself.
Plunger-type valves, knife gate valves, and custom made restrictors having an action similar to a “garotte” are also known. Similar problems are seen in the use of these valves as are exhibited with the muscle valves and the pinch valves. In particular, solids easily build up, forming crystalline gypsum, which then prematurely sets and causes further clogging of the apparatus.
Further, the prior art valves cannot easily reproduce exact settings which correspond to a given amount of restriction of the conduit. Further still, the prior art valves do not have interchangeable parts, nor are they adapted to be used with different sized conduits.
Accordingly, there is a need for an improved valve for a slurry mixing apparatus dispensing system and a method which promotes smoother, less air entrained slurry.
Another need is for an improved valve for a slurry mixing apparatus dispensing system which prevents build up of prematurely set slurry in the dispensing conduit.
Still another need is for an improved valve and method of use which is capable of continuously varying the flow of gypsum slurry from the mixing apparatus through the dispensing system to the production line.
A further need is for an improved valve system for a gypsum slurry mixing apparatus dispensing system and method of use which has parts that are readily adaptable to different sized conduits.
A further need is for an improved valve for a gypsum slurry mixing apparatus and dispensing system and method of use which provides an easily accessible mechanism for changing the volume of slurry emitted from the dispensing system.